Every day, thousands of people who are consumed by the nation’s opioid epidemic connect on the popular discussion website Reddit.

They swap advice on getting high and offer encouragement to those who have managed to stay clean or are teetering between recovery and relapse. Addicts lament the deaths of fellow users who have suddenly stopped posting. And until last week, buyers and sellers could easily find each other, relying on coded messages that communicated their intent.

Reddit banned the forum, known as opiaterollcall, last week but would not disclose what led to its closing. Another forum to buy opiates quickly sprang up to replace it; Reddit banned that one, too.

They were just small parts of one of the world’s largest online communities. But the dispatches left behind tell a surprisingly intimate story about the tenacity of the crisis, the trajectory of the addicted and Reddit’s role in facilitating access to drugs tied to the mounting toll across America.

Among the victims was Rachel Frazier, a former nurse and mother of a young son, now 3. Last June, she offered advice about medicine for withdrawals on a Reddit opiate support group that is simply called “opiates.” In September, she posted three times on the now-banned drug-buying forum opiaterollcall. She had recently moved to Mansfield, Ohio, from Texas with her husband, Jason.

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Rachel Frazier, a former nurse and mother. In September, she posted three times on a now-banned drug-buying forum, “opiaterollcall,” and died soon after from an overdose.

“Just moved to 419,” she wrote on Sept. 3, referring to her area code. Signaling that she wanted drugs, she wrote, “Looking for friends.”

Two weeks later, her husband found her dead in their living room, slumped over an ottoman with an unopened can of Diet Coke nearby. Relatives said a medical examiner determined that Ms. Frazier, 35, had overdosed on heroin and carfentanyl.

Her death was memorialized on Reddit by someone with the user name UhhImJef, who wrote about having spoken to Ms. Frazier just the day before. “We lost another,” the post read. “We’ll miss you sweets.”

Like others on the Reddit forums, Ms. Frazier had a longstanding addiction, her family said in interviews, and she procured painkillers in a variety of ways, not just online. The relatives said it was not clear when she switched to heroin, who provided the lethal dose, or if she got the drugs that killed her by way of Reddit.

It is just one of many places to find drugs online. In recent years, some of the biggest black markets have been on the so-called dark net, where buyers can place mail orders using Bitcoin and other virtual currency to operate with anonymity. AlphaBay, one of the largest such markets, was shut down two weeks ago, after a law enforcement crackdown.

On the public internet, drug buyers have been able to use sites over the years including Craigslist and Topix, forums on Google Groups and, more recently, mobile apps like Snapchat, Instagram and other social media platforms.

In a statement, a Reddit representative said that using the site “as a marketplace for illegal goods and services will get users and communities banned from Reddit.”

The company said it evaluates violations of its user agreement case by case, but did not explain why the opiate-buying forum had escaped detection from Reddit and law enforcement for at least four years. Reddit also hosts similar groups involving ecstasy and cocaine that have not been banned.

While police departments and federal officials are often aware that the public sites are used for drug sales, they tend to focus on major suppliers — like clinics flooding the market with pain pills — rather than low-level individual sellers.

“Given the volume of internet activity and the many ways to communicate online, it is obviously difficult to police everything, given limited resources,” Rusty Payne, a spokesman for the federal Drug Enforcement Administration, wrote in an email. “For DEA, this means going after the biggest fish.”

Even with tougher policing, online selling can be very resilient.

“When you shut it down, it will pop back up and it will have evolved into something new,” said Dan Palumbo, research director of Digital Citizens Alliance, an organization dedicated to internet safety issues. “It will be more efficient, more effective and it will probably be harder to track.”

Reddit hosts more than a million groups it calls subreddits, which are organized around a broad range of topics — bicycling, books, frugality, health, music, politics, “The Simpsons.” The site is a staunch defender of free speech, but administrators have sometimes banned forums that become controversial.

A group called “jailbait” — it contained provocative images of teenagers — led to a ban of “suggestive or sexual content featuring minors” in 2011. The company also shut down a group called “beatingwomen,” which glorified violence against women. Last year Reddit banned two so-called alt-right subreddits for repeatedly posting personal information that could lead to harassment. It took no action, though, against a subreddit organized around gun sales, which drew scrutiny after a 2014 Mother Jones article suggested that some arms dealers sought to exploit a federal background check loophole.

The opiate-buying forum that Reddit shut down last week offered a state-by-state directory where people could post their area code to show where they wanted to make a deal. Details of where and when to meet up were worked out in private messages, phone calls or texts.

Several people who use the forum said it felt safer to them than buying drugs on the street in rough parts of town.

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A series of screen grabs from the opiate-buying forum that Reddit shut down last week.

Reddit’s mostly anonymous users, called redditors, could vouch for people they met buying or selling drugs online. They typically posted ratings, with plus signs, stars and endorsements like “best friend” or “great friend.”

Users were warned not to post information that violates Reddit rules against sourcing illicit substances. But the “opiaterollcall” forum highlighted the ease of finding pain pills, heroin and sometimes fentanyl, the especially deadly drug that has accounted for a nationwide escalation in overdoses.

In messages to The New York Times, someone with the user name DopeAmene acknowledged assisting Ms. Frazier in getting opiates soon after she moved to Ohio. The user described serving — for her and others — as a broker.

“I took them to the person who did sell them,” DopeAmene wrote in a private message on Reddit. “And usually I would get some opiates for taking them.”

Tracey Helton Mitchell, a volunteer moderator of the “opiates” discussion group, doesn’t sugarcoat what’s going on: People talk about drug use in Reddit forums. And members of her group die every month, she says.

By Reddit standards, the forums are tiny — about 13,000 unique users posted on the buying and selling group over the past four years, some of them overlapping with the nearly 38,000 subscribers to the forum Ms. Helton Mitchell moderates. She distances herself from the drug-buying forum. Her group — which shares stories about scoring drugs and getting high, and discusses struggles with relapse, recovery and family — can save lives, she said in an interview.

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Tracey Helton Mitchell, a heroin user who has been clean for 19 years, is a moderator on the “opiates” discussion group on Reddit.CreditAndrew Burton for The New York Times

“Some people might see it as a dysfunctional one, but it’s a support group,” said Ms. Helton Mitchell, a former heroin addict. “People can’t get clean when they are dead.”

On the forum, she and others try to dispel myths, like the one suggesting that injecting someone with cocaine or speed after an overdose could save them. They offer warnings, such as a recent one saying that a batch of cocaine in the San Francisco area had been cut with fentanyl. Sometimes they send each other clean needles, or naloxone, the drug that can revive someone after overdosing.

Ms. Helton Mitchell, who lives in the San Francisco Bay Area, says she has sent out more than 500 packages of naloxone, sold under the brand name Narcan among others, and kept a tally of lives that have been saved — at least 238.

One of the packages went to a Reddit user calling herself jelllly, who wrote a tribute to Ms. Helton Mitchell two years ago. She “has done it again,” it said. “Sent me Narcan in the mail a couple months ago and I had to use it tonight. Along with breathing exercises, I revived a very close friend. So here’s to you, Tracey, God bless you!!!”

The post was from Shana Greger, a Reddit regular from Pennsylvania. Her comments on various forums provide a snapshot of her interests: hockey, the Pittsburgh Penguins, TV shows she liked to binge watch in withdrawal (“Nurse Jackie,” “Breaking Bad” and “Shameless,” to name a few).

Ms. Greger’s father discovered she was part of the “opiates” and “opiaterollcall” groups a few days after she overdosed last summer in State College, Pa. He had been searching online for some of her poetry, hoping to gather it up to publish. He found one poem on Reddit, and then her other posts.

Ms. Greger, 25, began stealing her mother’s pain pills as a teenager. Later, while working in coffee shops, managing a sandwich shop and advising her friends on the best new bands, she became consumed by addiction, according to her father.

The day after she was released from rehab last May, she congratulated another user in the opiates support group for 30 days of sobriety.

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Shana Greger, who died from an overdose in State College, Pa., last summer. Her father found one of her poems on Reddit, then found other posts.

“nice! good to see you’re doing well,” she wrote. “I’m 16 days clean from everything. ... I was in detox for 3 days and i feel fine. feels good to not have to take something every day.”

Eleven days later she commented about her life as a barista. She posted four times on the forum the next day. She died the day after.

Her father shared the news on the “opiates” group. While it had given her many friends and people to talk to, he wrote, he said he also believed they had helped his daughter find drugs. Some members objected; others told him about the drug-buying “roll call” forum.

Ms. Greger had posted there about 30 times in the six months before she died, looking for drugs and vouching for sellers in four states.

The police in State College tried to find out who sold her the heroin and fentanyl that killed her, but were not successful. It is not clear if she connected with the supplier through Reddit.

Mr. Greger says it doesn’t really matter how his daughter got the drugs. Initially shocked at the ease of acquiring them through Reddit, he now doesn’t think shutting down forums will make much of a difference.

“They will just move someplace else,” he said.

Correction:

An article on Friday about communities of drug users on the online forum Reddit misstated the year the website banned a section containing provocative images of teenagers. It was banned in 2011, not “about two years ago.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Online Corner Offers Glimpse Of Opiate Grip. Order Reprints | Today’s Paper | Subscribe